The invention relates to an apparatus for detecting the carbon dioxide content of air. Furthermore, the invention relates to a method for generating a gas measurement value representing the carbon dioxide concentration in air.
The detection of carbon dioxide is of great interest for a series of applications. Examples include the assessment of air quality in interiors, energy-efficient driving of air-conditioning systems or the monitoring of purified air. The aim of detecting carbon dioxide may be to increase comfort. However, it is also possible to achieve considerable energy savings under certain circumstances.
Thus, in the case of a well-insulated building, for example, almost half of the energy required for air-conditioning can be saved by demand-conforming air-conditioning. In this case, the demand is oriented, inter alia, toward the carbon dioxide content of the air. In the automative sector, too, demand-conforming ventilation and air-conditioning of the interior of the automobile is advantageous. An estimated value for the reduction of consumption for air-conditioning is 0.3 l per 100 km.
Carbon dioxide occurs in a concentration of approximately 380-400 ppm under normal ambient conditions in air. On the basis of this base concentration, a sensor for carbon dioxide has to be able to detect increased concentrations up to 4000 ppm, for example. What is problematic here is that the carbon dioxide molecule is a linear, symmetrical molecule and an electric dipole moment, which can bring about a sensor signal in various transducer principles, is therefore absent. Furthermore, the molecule is chemically very unreactive.
Currently very successful methods for determining the concentration of carbon dioxide can therefore be found primarily in the field of optical spectroscopy. In this case, use is made of the fact that carbon dioxide absorbs light in specific wavelength ranges, for example at a wavelength of approximately 4.3 μm. This enables an accurate and selective measurement of the concentration of carbon dioxide. The chemical reactivity of carbon dioxide is unimportant in this case. What is disadvantageous about optical spectroscopy, however, is the complex construction of the measurement systems and the considerable outlay required for evaluating the measured spectra. This ultimately leads to comparatively large and expensive measurement systems.
Solid-state sensors such as semiconductor gas sensors, for example, avoid the disadvantages of the optical measurement systems. They are small, can be produced by mass production extremely inexpensively in comparison and require less complex signal evaluation. What is disadvantageous about solid-state sensors, however, is that they rely on a certain reactivity of the molecules to be measured and at the same time, however, detect all molecules having indeed a certain reactivity. To put it another way, the solid-state sensors have a low selectivity. This makes it difficult primarily to measure not very reactive species such as carbon dioxide using such sensors, since they usually react very greatly to hydrocarbons or ozone.
The array of potential disturbing gases is extensive in this case. It comprises nitrogen dioxide (NO2), carbon monoxide (CO) and hydrogen (H2), ammonia (NH3), ethanol or hydrochloric acids (HCl), nitrogen monoxide (NO), sulfur oxides (SOx), carbon oxide sulfide (COS), nitrous oxide (N2O) and hydrogen cyanide (HCN), water (H2O) and also organic gases such as methane, ethane, ethene, acetylene and other hydrocarbons such as formaldehyde (CH2O). Further disturbing gases include amines (NH2R1, NH1R2, NR3), amides (RC(O)NH2, RC(O)NHR′, RC(O)NR′R), acrolein (C3H4O) and phosgene (COCl2), aromatics such as benzene (C6H6), ethylbenzene, chlorobenzene, toluene, xylene, styrene and phenol (C6H6O). Furthermore, there is ozone (O3), the large group of VOCs (volatile organic compounds).
These gases in some instances already occur in normal ambient air, for example ozone. Further sources of gases are fires, cigarette smoke, human activity, the use of chemical agents such as cleaning agents, exposed foodstuffs or technical devices such as printers. Road traffic and even weather conditions also lead to the occurrence of gases.
The document by H.-E. Endres et al., “A capacitive CO2 sensor system with suppression of the humidity interference”, Sensors and Actuators B 57 (1999), 83-87, discloses a CO2 sensor based on the principle of a capacitance measurement. In the case of the capacitive sensor disclosed, an additional humidity sensor is used to generate a humidity signal.
A potential-controlled humidity sensor that can be used for this purpose is known from EP 1 176 418 A2, for example. The potential-controlled humidity sensor has a gas-sensitive region which can be polarized independently of humidity. Furthermore, the gas-sensitive region has a relative permittivity that is dependent on the humidity.
What is disadvantageous about the capacitive sensors from the document by H.-E. Endres et al. is that heating of the sensor is necessary. This heating permanently consumes energy. Furthermore, the increased temperature of the sensor relative to room temperature also influences the dynamics of the interactions with surrounding gases, that is to say in other words alters the cross-sensitivities with respect to target and disturbing gases with respect to other sensors that are heated to a lesser or greater extent.